Yoghurt production

Yoghurt production

Yogurt is a fermented milk product that contains the characteristic bacterial cultures Lactobacillus bulgaricus and Streptococcus thermophilus in a ratio of 1 : 1. Streptococcus thermophilus  produces mainly acid from lactose in the milk causing the pH to drop to about 4-5 from about 7.0.  This drop in pH causes the milk to coagulate. Lactic acid gives yoghurt its sour taste and limits the growth of spoilage bacteria. Lactobacillus bulgaricus generates more organoleptic compounds, particularly acetaldehyde providing the flavour.  Proteolytic enzymes and extracellular polymers from the bacteria also aid protein-gel formation. Yoghurt can be pasteurized to improve storage-life or remain ‘live’, having probiotic qualities. Yoghurt is flavored usually with fruits. Full fat yogurt must contain not less than 3.25% milk fat, low fat yogurt contains not more than 2% milk fat, and nonfat yogurt contain less than 0.5% milk.

Ingredients

The main ingredient in yogurt is milk. The type of milk used depends on the type of yogurt – whole milk for full fat yogurt, lowfat milk for lowfat yogurt, and skim milk for nonfat yogurt.

Stabilizers such as alginates (carageenan), gelatins, gums (locust bean, guar), pectins, and starch may also be used in yogurt to improve the body and texture by increasing firmness, preventing separation of the whey (syneresis), and helping to keep the fruit uniformly mixed in the yogurt.

Sweeteners, flavors and fruit preparations may also be added in yogurt.

Bacterial Cultures

The starter cultures in yogurt are Lactobacillus bulgaricus and Streptococcus thermophilus. The function of the starter cultures is to ferment lactose (milk sugar) to produce lactic acid. The increase in lactic acid decreases pH and causes the milk to clot, or form the soft gel that is characteristic of yogurt. The fermentation of lactose also produces the flavor compounds that are characteristic of yogurt. Lactobacillus bulgaricus and Streptococcus thermophilus are the only 2 cultures required to be present in yogurt.  But other bacterial cultures, such as Lactobacillus acidophilus, Lactobacillus subsp. casei, and Bifidobacteria may be added to yogurt as probiotic cultures for improving lactose digestion, gastrointestinal function, and stimulating the immune system.

Steps in Yogurt Making

1.      Adjust Milk Composition & Blend Ingredients

2.      Pasteurize Milk

3.      Homogenize

4.      Cool Milk

5.      Inoculate with Starter Cultures

6.      Hold

7.      Cool

8.      Add Flavors & Fruit

9.      Package

1. Adjust Milk Composition & Blend Ingredients

Milk composition may be adjusted by adding dry milk powder to achieve the desired fat and solids content. Ingredients such as stabilizers are also added at this time.

2. Pasteurize Milk

The milk mixture is pasteurized at 85°C for 30 minutes or at 95°C for 10 minutes. High heat treatment is used to denature the whey proteins. This allows the proteins to form a more stable gel and prevents separation of the water during storage. The high heat treatment also reduces the number of spoilage organisms in the milk.

3. Homogenize

The blend is homogenized to mix all ingredients thoroughly and improve yogurt consistency.

4. Cool Milk

The milk is cooled to 42°C, the ideal growth temperature for the starter culture.

5. Inoculate with Starter Cultures

The starter cultures (Lactobacillus bulgaricus and Streptococcus thermophilus) are added.

6. Hold

The milk is held at 42°C until a pH 4.5 is reached. Fermentation results in the formation of a soft gel and the characteristic flavor of yogurt. This process take several hours.

7. Cool

The yogurt is cooled to 7°C to stop the fermentation process.

8. Add Fruit & Flavors

Fruit and flavors are added at different steps depending on the type of yogurt. For set style yogurt the fruit is added in the bottom of the cup and then the inoculated yogurt is poured on top and the yogurt is fermented in the cup. For swiss style yogurt the fruit is blended with the fermented, cooled yogurt prior to packaging.

9. Package

The yogurt is packaged

Health benefits of Yoghurt

Yogurt has been consumed by humans for hundreds of years, it is highly nutritious.  It contains high levels of calcium, Vitamin B12 and riboflavin, phosphorus, magnesium potassium, Vitamin D, etc.  It is a rich source of protein. Yoghurt contain probiotics that benefit digestive health and immune system.

 References

1. Industrial Microbiology: An Introduction, M J. Waites, N L. Morgan, J S. Rockey, G Higton
2. Modern Industrial Microbiology and Biotechnology, Nduka Okafor, Science Publishers
3. http://www.milkfacts.info/Milk%20Processing/Yogurt%20Production.htm